1. Field of the Invention
The present invention relates to cancellation of interfering signals for use in a power-inversion adaptive array.
2. Description of the Related Art
Adjacent channel interference and co-channel interference are the important problems of the mobile communications systems and digital microwave transmission systems using phase shift keying (PSK) and quadrature amplitude modulation (QAM) techniques. In the case of high speed digital transmission, in particular, frequency-modulated signals can be regarded as a narrow band interfering signal which can be easily cancelled by the use of linear and nonlinear filters. However, the signal will suffer from wideband interference from adjacent digital channels.
While the cancellation of wideband interference is difficult, a power-inversion adaptive array can be used for this purpose if the interfering signal is stronger than the desired signal by combining the interfering signals from diversity branches in opposite phases as described in a paper "The Power-Inversion Adaptive Array:Concept and Performance", R. T. Compton, IEEE Transactions, Vol. AES-15, No. 6, November 1979.
Although the power-inversion adaptive array is capable of cancelling interference if the D/U ratios (ratios of desired to undesired signals in decibels) of the diversity branches are negative, interference cannot be completely cancelled if there is a difference between the D/U ratios of the diversity branches. In radio transmission systems where the transfer coefficients of communication channels are constantly changing due to fading, the D/U ratios of the diversity branches have a low likelihood of having equal values.
On the other hand, with conventional LMS (least mean square) interference cancellers, the mean square error adopts a downwardly convexed quadric surface so that the partial differential coefficients of second order of weight coefficients assume a maximum eigen value. As a result, with an increase in the eigen value the plane of the decision error takes the shape of a sharper quadric surface. The update coefficient .mu. of the LMS algorithm is normally set at a sufficiently small value. However, as the quadric error surface becomes sharper, the amount of weight coefficient to be updated increases significantly so that adaptive convergence is no longer possible. It is generally recognized that the update coefficient .mu. must satisfy the condition 0&lt;.mu.&lt;2/.lambda..sub.max. (where .lambda..sub.max is the upper limit of eigen values). For a strong interfering signal, the upper limit .lambda..sub.max increases correspondingly and if the update coefficient .mu. is set at a low value to satisfy the above condition, the convergence speed will decrease. Therefore, the LMS interference canceller cannot be used in applications where the D/U ratio is negative.